A typical data communication network is configured to operate according to a single predetermined protocol, e.g., an Ethernet protocol, a token ring protocol, other LAN protocols, or an isochronous protocol. An example of an Ethernet system is an implementation known as 10 Base T which is described in the draft Nine supplement to IEEE standard 802.3, dated Nov. 15, 1989. Other examples of data communication protocols are X.25, and the Token Ring System, described for example, by IEEE Standard 802.5. Both Ethernet and token ring systems convey data in packets but each uses a different media access method.
As shown in FIG. 1A, in a packet system, data is transferred in a plurality of packets 12a, 12b which can be either constant-sized or variable-sized. Each packet includes a field of data 14a, 14b which may be preamble information 16a, 16b housekeeping information such as data source information, data destination information, and the like 18a, 18b and a frame end marker 20a. As seen in FIG. 1A, because the fields provided for data 14a, 14b are not substantially continuous, the packetized scheme of FIG. 1A is not isochronous but "bursty" in nature.
In a token ring system, a node is permitted to transmit data only after receipt of an electronic "token." As depicted in FIG. 1B, a first station may transmit a token 22 which is received 24a by second station whereupon the second station may begin transmission of data 26a. After a period of data transmission, the second station transmits the token 22b which is received by a third station 24b that can then begin its own transmission of data 26b. As seen in FIG. 1B, because data transmission is synchronized with the occurrence of an event (the arrival of a token), the token ring system is not an isochronous data transfer system.
FIG. 1C schematically depicts isochronous data transfer. In general terms, isochronous data is data which is non-packetized and of indeterminate, potentially continuous duration. In isochronous data transfer, the data transfer is initiated, such as by initiating a telephone conversation or beginning a video camera transmission 30. After the data transfer or connection is initiated, transmission of the data, possibly accompanied by transmission of housekeeping information (such as destinations, audio or video trimming, and the like) is provided substantially continuously for an indeterminate period such as until termination of the connection 32. Although it may be that not every bit transferred represents a data bit (since "housekeeping" bits may also be transferred), the transfer of data is substantially continuous in the sense that there are no substantial periods during which no data bits are transferred. It is possible that the data being transferred is "Null" data such as silence during a telephone conversation or transfer of a blank video image. One type of isochronous data transfer protocol is the Fiber Distributed Data Interface-II (FDDI-II) as described, for example, in FDDI-II Hybrid Multiplexer, Revision 2.4, dated Mar. 25, 1991.
Previous systems which were configured to use only a single-type protocol had the disadvantage that it was not possible to operate a mixed-protocol or "mixed-environment" system. Also when upgrading a network system, it was necessary to upgrade the entire system and it was infeasible or wasteful to upgrade only part of the system (such as only some of the nodes or such as upgrading nodes without upgrading hubs or upgrading hubs without upgrading nodes). Additionally, when a system or system components were installed, or repaired it was necessary for the installing personnel to be familiar with the particular single protocol for which the network was configured and to make such installation, upgrade, or repair in accordance with such a single protocol. Furthermore, it was necessary that apparatus connected to the system be configured for exclusive operation in accordance with the predetermined single protocol.